This invention relates to a Magnetic Resonance Imaging (MRI) system. More particularly, this invention relates to radio frequency (RF) coils used in MRI systems for transmitting and/or receiving RF signals.
MRI scanners, which are used in various fields such as medical diagnostics, typically use a computer to create images based on the operation of a magnet, a gradient coil assembly, and at least one radiofrequency coil. The magnet creates a uniform main magnetic field that makes nuclei, such as hydrogen atomic nuclei, responsive to radiofrequency excitation. The gradient coil assembly imposes a series of pulsed, spatial magnetic fields upon the main magnetic field to give each point in the imaging volume a spatial identity corresponding to its unique set of magnetic fields during the imaging pulse sequence. The radiofrequency coil(s) creates an excitation frequency pulse that temporarily creates an oscillating transverse magnetization that is detected by the radiofrequency coil and used by the computer to create the image.
Generally, very high field strength is characterized as greater than 2 Tesla (2T). Higher magnetic field strength imposes challenges on the RF coil, such as balancing inductance and capacitance at relatively higher frequencies, i.e. greater than 64 MegaHertz (MHz). At very high magnetic fields, and therefore very high Larmor frequencies, standard birdcage coils with moderately narrow rung copper strips have relatively high inductance requiring very low capacitor values in order to resonate the coil. This is problematic for a number of reasons. First, high currents through small value capacitors will have very high voltage potential across them which can result in a local stray electric field that dissipates RF power in the form of heat in an imaging subject.
There are two types of electric fields associated with MRI. The first is due to time-varying B1 magnetic field present within the imaging subject and the second type is due to electric charges on the capacitors in the RF coil structure. When a NMR system is operating at a relatively high frequency range, for example above 100 MHz, significant radiation loss may occur. The increased radiation loss in high frequency ranges results in an increase in RF power used to generate the excitation and a resultant decrease in the signal-to-noise (SNR) of the signals received.
In one aspect, a radio frequency (RF) coil assembly for imaging a subject volume using a very high field Magnetic Resonance Imaging (MRI) system operable at substantially high frequencies is provided. The MRI system includes a plurality of conductors arranged cylindrically and disposed about a patient bore of the MRI system, a plurality of capacitive elements disposed between and connecting respective ends of the conductors, the plurality of conductors and plurality of capacitive elements forming a high band pass birdcage configuration, and a plurality of dynamic disabling switches, each dynamic disabling switch electrically coupled in parallel with a respective capacitive element to form a parallel resonant circuit.
In another aspect, a magnetic resonance imaging (MRI) system is provided. The MRI system includes a radio frequency (RF) coil assembly for imaging a subject volume using substantially high frequencies. The RF coil includes a plurality of conductors arranged cylindrically and disposed about a patient bore of the MRI system, a plurality of capacitive elements disposed between and connecting respective ends of the conductors, the plurality of conductors and plurality of capacitive elements forming a high band pass birdcage configuration, and a plurality of dynamic disabling switches, each dynamic disabling switch electrically coupled in parallel with a respective capacitive element to form a parallel resonant circuit.
In a further aspect, a TEM resonator is provided. The TEM resonator includes a plurality of conductors arranged cylindrically and disposed about a patient bore, a plurality of capacitive elements disposed between and connecting respective ends of the conductors, the plurality of conductors and plurality of capacitive elements forming TEM resonator configuration, and a plurality of dynamic disabling switches, each dynamic disabling switch electrically coupled in parallel with a respective capacitive element to form a parallel resonant circuit.
In still a further aspect, a method for operating a RF coil in a very high field Magnetic Resonance Imaging (MRI) system operable at substantially high frequencies is provided. The method includes arranging a plurality of conductors cylindrically around a patient bore of the MRI system, connecting a plurality of capacitive elements between respective ends of the conductors, the plurality of conductors and the plurality of capacitive elements forming a high band pass birdcage configuration, connecting a plurality of dynamic disabling switches in parallel with a respective capacitive element to form a parallel resonant circuit, each dynamic disabling switch including a diode, and connecting a switching bias to a second end of said dynamic disabling switch, the switching bias configured to forward bias and reverse bias said diode.